An efficient power decoupling topology circuit based on a novel three-port three-switches flyback series circuit

Both filter inductors, electrolytic capacitors, and radiators play a significant role in the inverter of a PV (Photovoltaic) power generation system. These three parts are the largest in an inverter, which affects the performance of the inverter. Aimed to improve the power density of a single-phase PV grid-connected inverter with a decoupling function. This paper derived the control principle that can reduce the volume of the inductor, decoupling capacitor, and the loss of the switching device to begin with the mathematical function of power processing of the filter inductor. And then, the authors deduced a boost-type power decoupled single-phase inverter topology. Based on a novel three-port three-switches flyback series circuit, this paper proposed an efficient power decoupling topology circuit for extracting the maximum power density of a single-phase grid-connected PV inverter. Finally, this article operated the simulation and experiment. Both the simulated and experimental results verified that the proposed method works well.


Introduction
Single-phase grid-connected inverters are widely attractive in distributed photovoltaic power generation, wind power generation, and other fields.In these application fields, new requirements are put forward for inverter devices achieving higher power density under high efficiency [1,2].The nonlinearity, randomness, and uncontrollability of wind speed and solar irradiance make inputting voltage of the inverters deviate from its given or rated value [3,4].The factors will lead to a significant power loss.To solve this problem, improve the adaptability to a wide range of input voltage and optimize the system reliability, flyback inverter, multistage inverter, and boost single-phase inverters with input voltage regulation functions have received wide attention [5,6].
Nowadays, there are three types of inverters in PV systems, including string inverters [7], ac/AC (Alternating Current) modules [8], and centralized inverters [9].AC module has gotten a lot of attention from the industry and researchers due to its merits: 1. an improved system efficiency; 2. enhanced modularity and flexibility; 3. a plug-n-play operation; 4. improved strategy; section V illustrates the simulation and experiment performed in this paper as well as their analysis; finally, section VI concludes the whole research.

Traditional single-phase inverter topology and its gridconnected flyback inverter
During a definite switching period in the topology of this paper, AC power refers to the charging or discharging of an inductance; DC power refers to the power transmitted directly from the power source to the load end in this period.Consequently, the sum of AC power and DC power is the output power in this period.
As shown in Fig 1 below, DC power P DC denotes the power transferred directly from the power source to the load without the volume or loss of power converter components; AC power P AC refers to the one transmitted to the load circuit when reactive devices finished processing or converting.[41].The inputting power P PV is one constant value decided via the MPPT method.The output power P out (t) is a variable that varies with time and consists of the AC and DC parts.Supposing this inverter as lossless, the injected current and the grid voltage to be sinusoidal in-phase waveforms, the DC aspect of the outputting power is the inputting power, and the AC aspect fluctuates in a double-line frequency.Consequently, the real-time inputting power and instantaneous outputting power are as expressions (5,6) below:

Traditional grid-connected flyback type inverter
The system processes the fluctuation aspect of the outputting power passing through one buffer to achieve a constant DC on the inputting side.This instantaneous power in the buffer equals Eq (7) below: Different from the meanings of P DC in Fig 1 (where it refers to the power transferred directly from the power source to the load without the need for a power converter or loss of power).The parameter P PD in Eq (3) represents the power delivered by decoupling circuits, while the abbreviation PD stands for Power Decoupling.
The system puts a big electrolytic capacitor C PD across the PV to control the voltage fluctuation in one passive power decoupling circuit.Nevertheless, the electrolytic capacitor is commonly sensitive to the environment's temperature and could go down the overall reliability of the inverter.
Fig 3 below illustrates a traditional flyback inverter fitted with a power decoupling circuit.The technology of active power decoupling is the other to compensate for power differences using active switches and long-lifetime thin film capacitors.If the inputting power is more than the outputting one, then the system will transport the extra energy to the decoupling capacitor from the PV.While the inputting power is less than the demanded grid power, this energy of the decoupling capacitor transports to the outputting end.In the three-port power decoupling circuit, by using a port for the power decoupling but applying the rest two ports for obtaining the inputting power and conveying this power into the outputting port.
There are two operating states in the inverter based on this power difference among the instantaneous outputting and inputting powers, including State I and II.On the one hand, in State I, the inputting power is less (at least no more) than the outputting power, and that decoupling capacitor is discharging.Then again, during State II, the inputting power is more (at least no less) than the outputting power, and the decoupling capacitor charges via absorbing the energy.Because this decoupling circuit only deals with the pulsating power, so average power of the decoupling topology should be zero.The presented AC module is an improved single-phase flyback type inverter, which is a modified circuit deduced from the traditional flyback inverter by integrating an active power decoupling circuit and another transformer winding.The proposed inverter with three switches can get maximum power from the PV based on the MPPT method, input the sinusoidal current into the grid, and compensate for the differences between the outputting and inputting powers through one little thin-film capacitor.Consequently, just operating switches S 1 , S 2 , and S 3 can readily achieve those functions.
This power decoupling circuit includes a decoupling capacitor C 1 and a diode D 1 .There is no individual switch to deal with the pulsating power in the proposed decoupling topology.Nevertheless, the system uses a flyback main switch S 1 to keep the pulsating energy and the PV energy in that transformer in storage.Placing a buffer capacitor across S 1 can decrease the electromagnetic noise and supply some soft-switching conditions.Applying two secondary-side diodes D 2 , and D 3 , as well as two switches S 2 , and S 3 , connected in series, can transfer the power to the grid from the PV using a proper transformer winding.The presented inverter topology filters some switching frequencies using an LPF (Low-Pass Filter) and transfers a low THD (Total Harmonic Distortion) current into the grid.
The inverter proposed in this article has three merits: (1). it is a plain architecture with just one capacitor and only one diode equipped with the secondary winding of the transformer for implementing the function of the power decoupling; (2). it can realize the MPPT, operate the power decoupling using only three switches, and put the sinusoidal current into the grid; (3). the circuit performs in a DCM (Discontinuous Conduction Mode), which obtains merits from the soft-switching technology and just one easy controlling method.

Operation modes
There are five modes of inverter operation within every switching cycle, where nab symbolizes the turn-ratios of the transformer: n a /n b (a, b = 1, 2, 3, 4).Fig 5 below shows the vital waveforms of the inverter in one switching cycle.Because the switching frequency of the inverter is much bigger than the power system frequency, the referencing current and the grid voltage are nearly stable within one switching cycle.This paper supposed that all switches are in the state of off before the first sub-interval; in addition, the voltage of the decoupling capacitor V C1 equals V c-a ; furthermore, the authors also supposed the voltages of the grid are at the upper half period.
Accordingly, the following parts describe all five operation modes of the proposed inverter in Figs 6-10: Fig 6 below shows the equivalent circuit of operation mode 1.The magnetizing inductance of the transformer L m and the buffer capacitor C S1 are both in the state of resonance before mode 1.At time t 0 , turning off switches S 3 and turning on S 1 and S 2 , when that voltage bypassed switch S 1 , meets the minimum in its resonance.Consequently, the circuit will turn on switch S 1 close to the condition of the ZVS (Zero Voltage Switching).Switch S 2 conducts no current despite turns on it because this series diode D 2 is reversed-biased.Therefore, the circuit turns on switch S 2 under the condition of the ZCS (Zero Current Switch).Applying the sum of the decoupling capacitor and the PV cross magnetizing inductances and the transformer leak in this mode is vital.Since the proposed circuit works in the DCM(Discontinuous-Conduction Mode), the current in the transformer linearly increases from the point of zero can be formulated as below: Mode 1 will not stop until turning off switch S 1 at the time of t 1 .The system delivers some portion of the energy of the decoupling capacitor into the magnetizing inductance in the transformer.Consequently, this voltage will go down to V c-b from V c-a when mode1 ends.Thus, we can describe the voltage of the decoupling capacitor at the end of this mode as Eq (9) below: ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi In the expression above, Þf d 1 means the maximum value of the magnetizing inductance current in the specific transformer; and d 1 ¼ ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffiffi q denotes the duty cycles of switch S 1 .
• MODE 2 (PERIOD BEING t 1 < t < t 2 ) Fig 7 below gives the equivalent circuit of operation mode 2.
Because the system places capacitor C S1 across S 1 , switch S 1 's voltage smoothly increases and will be turned off in the condition of the AVS.Compared with a state of the art that limits the voltage change by putting a large electrolytic capacitor CPD across a PV in the passive power decoupling, which could decline the reliability of the inverter due to the temperature sensitivity, the proposed model can ignore the smooth little magnitude increase of switch S1's voltage through using an active power decoupling.This increasing voltage starts from the value of zero and lastly ends with a value of V SS : This current of the leakage inductance in the transformer slowly goes down during mode2.and will be zero when mode 2. ends.The authors consider the magnetizing inductance constant because the magnetizing inductance is much bigger than the leakage one, and the lasting time of mode 2 is so short.Formulas (7,8) below respectively symbolize the voltage of switch S 1 and the leakage inductance current in the transformer: Where, q .
• MODE 3 (PERIOD BEING t 2 < t < t The outputting current of the magnetizing inductance at the end of mode3.could be related to its initial value from starting this mode.A minimum value of the outputting current in the transformer at the start of mode 3 is: • MODE 4 (PERIOD BEING t 3 < t < t 4 ) Fig 9 below denotes the equivalent circuit of operation mode 4.
At the time of t 3 , the circuit turned S 2 off and charged the decoupling capacitor using the rest energy in the flyback transformer via diode D 1 and the second winding of the transformer.The current of the magnetizing inductance from the starting of mode 4 equals as expression (11) below: When mode 4. ends, the voltage of the decoupling capacitor could increase to V c-c : In addition, formula (13) below represents the voltage of switch S 1 : • MODE 5 (PERIOD BEING t 4 < t < t 5 ) Fig 10 below denotes the equivalent circuit of operation mode 5.The system goes to this mode when the energy in the transformer completely discharges into the decoupling capacitor at time t 4 .
At this moment, resonance occurs among the transformer magnetizing and the buffer capacitor, as well as leakage inductance.Expressions (14,15) below, write the buffer voltage and the inductance current: In the equations above, Z 0 1 ¼ ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi p denotes the angular frequency of the grid voltage, and inductance the steady-state voltage of switch S 1 at this mode.There are five functional modules: a PLL(Phase-Locked Loop), voltage sensors, half-cycle detection, one outputting current controller, and an MPPT controller.As shown in this figure, the proposed circuit can measure some parameters, including the grid voltage V ac , the decoupling capacitor V C 1 , the PV current I PV , and the voltage of PV V PV .The controlling strategy designed here ensures the system can abstract the MPP in the PV and transfer this maximum power into the grid in good quality.

Control strategy
An MPPT controller can abstract the maximum power stored in the PV panel based on an incremental conductance algorithm.The proposed inverter works under the condition of the DCM.Turning switch S1 on can store the energy in the flyback transformer through the decoupling capacitor and the PV.It is better to determine the duty cycle d 1 of switch S1 based on the MPPT method to satisfy the definition (i.e., d 1 ¼ ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffiffi In this meaning above, L m stands for Transformer magnetizing inductance, and L l represents Transformer leakage inductance.L m and L l are inversely proportional to the Duty cycle of switch S1 (i.e., d 1 ) but proportional to the Maximum current of transformer output winding during mode 3 (i.e., i peak21 ), according to Fig 13,expression (5) and equation (9).Furthermore, as far as L m is concerned, it is inversely proportional to the Minimum current of transformer output winding during mode 3 (i.e., i peak22 ) but proportional to Subtraction of d2 (d3) and d1 (i.e., d' = d 2 − d 1 ) based on Fig 13,formula(10) and expression (16).
Employing a block of PLL can determine the phase angle of the voltage because the grid voltage should be in phase with the outputting current.Using this module also can recognize the voltage negative and positive half cycle.The inverter will control S 2 and quench S 3 when the grid voltage is in the positive half cycle.Conversely, it tends to turn switch S 2 off and control switch S 3 when the voltage locates at its negative half cycle.The duty cycle of switches (S 2 or S 3 ) equals a sum of d 1 and d' deduced by Eq (9):

Simulation results and analysis
This article built a simulating topology circuit with 750 Watts in MATLAB to verify the proposed method's performance.Table 1 below lists the main simulation parameters of the proposed topology.S1, the voltage of the S1 slowly increases owing to the buffer capacitor.Accordingly, the circuit will turn switch S1 OFF in the condition of the ZVS.
In addition, Fig 13 demonstrates the other simulation results from a different perspective:

Experimental results and analysis
To test the performance of the presented inverter, we implemented a paradigm inverter with the 100 watts shown in Fig 14 .The experiment selected an STM32F407VGT6 equipped with a 32-bit MCU of an ARM Cortex-M4 as the core in implementing this digital controller.In addition, the design adopted a simulator as an inputting power source.

Design Parameters Value
The voltage of the AC grid V AC 220V The frequency of the AC grid voltage f ac 50Hz The inputting voltage of the DC power supply V PV 60V The switching frequency f 50Hz The nominal outputting power 100W The turn-ratios of the transformer (n 1 : n 2 : n 3 : n 4 ) 1:1:4:4 The turn-ratios of the transformer L m 50μH The inductance of the filter L f 1mH The buffer capacitor of the switch S 1 C S1 4.7nF The capacitor of the filter C f 15μF The capacitor placed across PV C PV 35μF The capacitor the power decoupling C 1 80μF https://doi.org/10.1371/journal.pone.0305773.t002 Table 3. Parameters of the switches and diodes.voltages also can measure the output and input power.This implemented inverter reaches its maximum efficiency point of 91.2% for half of the rated power nearly while realizing the efficiency value of 88.8% at its rated one.

Algorithm efficiency and loss distribution
Fig 20 below illustrates the loss distribution of various significant components at the rated power.The loss of diodes, three switches, filter inductance, and flyback transformers is more than 90% of the whole losses.The most important source of the inverter losses is the core loss.Utilizing a larger wire width and a bigger core size could reduce the core loss and optimize the system's efficiency.Furthermore, some high-flux density(permeability) materials, just as noncrystalline or amorphous wire, can significantly increase the efficiency of the micro inverter while they will cost.
Under the same design conditions, we tested the heat dissipation of the switch inverter system using three different switching devices: F4-23MR12W1M1_B11, F4-100R06KL, and F4-30R06W1E3.Table 4 illustrates that the method proposed in this paper outperforms the traditional H-bridge topology in conduction loss, switching loss, total loss, and heat dissipation performance.
As shown in Table 4, this topology reduces the voltage stress on the switching tubes, which is beneficial for selecting high-performance switching tubes with low power levels and further reducing switching losses.According to the Pareto optimality principle, when other parameters remain unchanged, the conclusion that the new topology is superior to the traditional one was validated by comparing the AC power of the inductance and the heat dissipation of the switching devices between the traditional and proposed topologies.
Besides Table 4, we also created Fig 21 based on the data in Table 4 to enhance our understanding.Three different switching devices mentioned in the table have the same four columns in this figure.The first column represents Conduction loss/W, the second column represents Switching loss/W, the third column represents Total loss/W, and the fourth column represents Radiator volume ratio.This figure illustrates the same conclusion that the method proposed in this article outperforms the traditional H-bridge topology with four switches.

Conclusion
The grid-connected inverter is an essential component in a PV system, and its performance will be affected by filter inductance, a radiator, an electrolytic capacitor, etc.This article presented a new Boost-type power decoupling inverter based on the principle of power flow optimization to reduce the volume of the filter inductor, decrease the switching loss and improve the power density of the inverter.The proposed power can extract the maximum power point of the PV, deal with ripple power and transfer the low THD-type sinusoidal current into the grid only with three ports and three switches.The simulation and experimental results show that compared with the traditional single-phase inverter topology, under the same experimental conditions, decreasing the number of those active switches can improve the reliability of the micro-inverter through the presented method.The proposed compact, trusty, and economical inverter only have three disjunctors and an easy controlling strategy, so it is a good choice for a PV low-power decentralized application.Virtually this research can improve the reliability of the micro-inverter by reducing the number of active switches.Although the proposed inverter, a somewhat perfect candidate for the PV low-power decentralized application, is a compact, reliable, and economical method, it has shortcomings, such as plug-and-play operation and extended system efficiency.Therefore, these limitations are our future research.
As for a PV system with an MPPT, like the grid system utilized in our paper, the average voltage of decoupling capacitors increases as the MPPT controller increases its inputting power.The amplitude of the injected current, I m , increases compared to a reference voltage value.Much more grid power will be injected into the system.Therefore, the current I m is different from before, so the average voltage of decoupling capacitors is close to that reference value.Accordingly, the presented topology is more suitable for low to medium powers (up to moderately high) than for high powers.

Fig 2
Fig 2 gives one traditional grid-connected flyback type inverter[41].The inputting power P PV is one constant value decided via the MPPT method.The output power P out (t) is a variable that varies with time and consists of the AC and DC parts.Supposing this inverter as lossless, the injected current and the grid voltage to be sinusoidal in-phase waveforms, the DC aspect of the outputting power is the inputting power, and the AC aspect fluctuates in a double-line frequency.

3 )
Fig 8 below illustrates the equivalent circuit of operation mode 3. The voltage of the decoupling capacitor is unchanged in this mode.Fig 8 shows that turning S 2 off at the time of t3 can terminate mode 3.During mode3, switch S 2 remains on, and by operating diode D2 and switching S 2 transfers some parts of the energy stored in the proposed flyback transformer into the grid.A peak value of the outputting current in the transformer at the start of mode 3 is:

Fig 11
Fig 11  illustrates a block diagram of the presented flyback inverter.There are five functional modules: a PLL(Phase-Locked Loop), voltage sensors, half-cycle detection, one outputting current controller, and an MPPT controller.As shown in this figure, the proposed circuit can measure some parameters, including the grid voltage V ac , the decoupling capacitor V C 1 , the PV current I PV , and the voltage of PV V PV .The controlling strategy designed here ensures the system can abstract the MPP in the PV and transfer this maximum power into the grid in good quality.An MPPT controller can abstract the maximum power stored in the PV panel based on an incremental conductance algorithm.The proposed inverter works under the condition of the DCM.Turning switch S1 on can store the energy in the flyback transformer through the decoupling capacitor and the PV.It is better to determine the duty cycle d 1 of switch S1 based Figs 12 and 13 show the simulation results.Fig 14 below illustrates some different current and voltage waveforms according to the genuine values of parameters.Via turning off switch

Fig 11 .
Fig 11.Block diagram of the controlling strategy of the designed inverter.https://doi.org/10.1371/journal.pone.0305773.g011 Fig 15 below illustrates the experimenting production of the outputting current, the grid voltage, and the voltage of the decoupling capacitor.A power analyzer of C.A.8335 surveyed the outputting current's THD is 3.5%.As shown in the figure, the output current is one sinusoidal with the same phase as the grid voltage.As shown in the figure, the output current is one sinusoidal signal with the same phase as the grid voltage.The voltage of the decoupling capacitor possesses one impulsing part at the double-line frequency, which has a 37V peak-topeak.This 37V voltage is superposition on an offset value of 110V.It ensures putting the power of the 100 Watts into the grid.Fig 16 below demonstrates the original output current of the inverter before filtering and the gate driving signals of switches S 1 and S 2 for the positive half-side cycle.Switching S 2 and turning off S 3 can produce a sinusoidal current in the output after filtering.The duty cycle in

D 2
&D 3 UF4008 t rr = 75ns, I f = 1A, V D = 1000V S 2 &S 3 STU7NB100 V DS = 1000V, R DS = 1.2O https://doi.org/10.1371/journal.pone.0305773.t003voltage be −V C1 + n 23 V out , where n 12 is the turn-ratios of the transformer n 1 /n 2 , and n 23 is n 2 /n 3 .Fig 18 below illustrates the dynamic response of the inverter presented in this article.When the PV voltage goes down to 40V from 55V, the ripple voltage of this decoupling capacitor goes down from 25 to 15V.Meanwhile, the output power decreases to 35W from 62.The controlling method can compensate for the voltage variation of the decoupling capacitor during four periods by improving the output power.

Fig 19
Fig 19 below represents the efficiency of the presented inverter with the output power changes.Dividing P out by P in can compute this efficiency, and multiplying corresponding currents by

Table 2
listed the critical parameters and corresponding values used in the circuit, and Table3specified those switches and diodes.

Table 4 . Comparisons between traditional H-bridge and the proposed topology.
The radiator volume ratio is the ratio between the two topological types, and the proposed method is the value before the colon in the ratio.https://doi.org/10.1371/journal.pone.0305773.t004